In pursuit of higher engine efficiencies, higher turbine inlet temperatures have been relied upon to boost overall engine performance. This can result in gas path temperatures that may exceed melting points of turbine component constituent materials. To address this issue, dedicated cooling air is extracted from a compressor section and is used to cool the gas path components in the turbine, such as rotating blades and stator vanes for example, incurring significant cycle penalties.
One method of cooling turbine airfoils utilizes internal cooling channels that include cooling holes to purge the cooling air. Typically, these cooling holes and significant cooling mass flow rates are required to provide the needed amount of cooling. In order to effectively cool the airfoils to protect against damage, there is a need to balance the amount of cooling flow used and the overall heat transfer capability. Some cooling channels have a very small surface area to receive the cooling holes. The cooling holes in these small areas provide an outlet to drive flow in the cavity for internal convection heat transfer and provide film cooling for the airfoil externally.
The small surface areas that are to include the cooling holes present manufacturing challenges. Prior configurations are limited by cooling hole manufacturing tolerances such as ligament distances between holes, sidewall distances between cavity sides, and backstrike distances. These tolerances only allow a few holes to be placed at these small cavity locations. Further, to pass the necessary amount of flow through the cavity, the diameters of the cooling holes had to be relatively large. This results in inefficient film cooling as film coverage is low.